1. Field of the Invention
The present invention relates to a wireless communication device and a wireless communication method that perform communications by use of a wireless frame containing a control channel and a data channel.
2. Description of the Related Art
In a cellular mobile communication system, cells as communication areas covered by respective base stations are located so as to be neighboring to each other or partially overlapped with each other. Each base station connects each mobile station to the mobile communication system by performing wireless communications with a plurality of mobile stations located within each corresponding cell. In this type of cellular mobile communication system, an important subject is to improve a throughput in the mobile station located at the cell edge. Generally, an error rate characteristic deteriorates in the cell edge, because the received power from the base station covering the cell is attenuated, and the mobile station is susceptible to interference by the signals from the base stations located in other cells in the periphery.
A method of solving such a problem is a method of giving, as for transmission signals to the mobile station at the cell edge, more of redundancy of the data than the transmission signals to other mobile stations. This is the method of transmitting the data repeatedly inserting the information having the same content with respect to the transmission signals to the mobile station at the cell edge. This method, however, causes the transmission frame to be upsized and leads to an increase of an overhead of the transmission frame. Further, even when fixing a transmission frame length, it follows that a substantial data transmission quantity decreases.
Another method is that the transmission data to the mobile station at the cell edge is subjected to error correction coding that exhibits the higher redundancy than the transmission data to other mobile stations. Proposed in regard to this method is a scheme (refer to Non-Patent document 1) that turbo encoding having a one-third (⅓) coding rate is used for the transmission data to the mobile station in the vicinity of the base station, and the low-rate turbo encoding having the coding rates such as ⅕ and 1/9 that are equal to or lower than ⅓ is used for the transmission data to the mobile station at the cell edge (refer to “3rd Generation Partnership Project, “Lower rate extension of channel coding to the rate <⅓”, 3GPP TSG RAN WG1 meeting #42bis (R1-051082), 2005-10, Agenda Item 8.7.”). With this scheme, even in the communications to the mobile station located at the cell edge in a transmission environment of very poor quality, it is expected to prevent the deterioration in the error rate characteristic and the decrease in the throughput by using the low-rate turbo encoding with the increased redundancy.
In the conventional arts described above, however, though capable of improving the decrease in the throughput with respect to a data channel for transmitting to the mobile station located at the cell edge, a problem about the transmission of a control channel still remains. This problem will hereinafter be explained with reference to FIGS. 14, 15 and 16. FIG. 14 is a diagram showing a frame format used for the conventional cellular mobile communication system. FIG. 15 is a diagram showing a coding switchover method corresponding to a cell location in the prior art. FIG. 16 is a diagram showing a transmitting/receiving state in the prior art.
The frame format as shown in FIG. 14 is used for the communications between the base station and the mobile station. The wireless frame is assembled by a pilot channel (PICH), a control channel (CCH) and a data channel (DCH). The coding switchover method proposed in the conventional art described above is a technique targeting on the data channel in the wireless frame. Generally, as for the control channel, convolutional coding having the coding rate “⅓” is used in common with the respective mobile stations.
FIG. 15 shows an example of such a case that a base station 500 covering a cell 510 performs the communications with a mobile station 501 located in the vicinity of the base station 500 and with a mobile station 502 located at the cell edge. In the conventional art, 16 QAM (Quadrature Amplitude Modulation) modulation method and the turbo encoding having the coding rate “⅓” are used for the data channel in the signals to the mobile station 501 located in the vicinity of the base station 500, and QPSK (Quadrature Phase Shift Keying) modulation method and the convolutional coding having the coding rate “⅓” are used for the control channel. Such a design is made that the transmission of the control channel totally exhibits a more preferable error rate characteristic than by the transmission of the data channel in the vicinity of the base station 500.
On the other hand, the communications with the mobile terminal 502 located at the cell edge involve such a change that the QPSK modulation method and the low-rate turbo encoding with its coding rate equal to or lower than the coding rate “⅓” are used for the data channel, and involve using, for the control channel, the QPSK modulation method and the convolutional coding having the coding rate “⅓” as they are. Namely, in the mobile station located at the cell edge, the data channel uses the coding having the larger redundancy than the control channel has, and therefore such a phenomenon might occur that the error rate characteristic of the control channel becomes lower than the error rate characteristic of the data channel at the cell edge.
In the conventional arts, the control channel contains control information for correctly demodulating and decoding user data allocated to the data channel, and hence, as shown in FIG. 16, if the mobile station 502 detects that an error occurs in the control channel, the mobile station 502 prompts the base station 500 to execute retransmission. Accordingly, in the conventional arts, it follows that the retransmission is repeated if the error frequently occurs in the control channel and that a decrease in transmission efficiency of the whole system is brought about.
For solving these problems, it is considered that the redundancy of the coding used for the control channel is set larger than the coding redundancy for the data channel. In this method, however, the control channel generally uses a format common to all the mobile stations, and therefore, when increasing the redundancy for the control channel and upsizing the control channel, it follows that the whole transmission frame gets excessively upsized. Further, even when fixing the transmission frame length, it follows that the substantial transmission quantity decreases due to the increase in the overhead of the control channel. Hence, this type of method results in increasing the overhead of the transmission frame.